1. Field of the Invention
This invention relates to an apparatus for detecting foreign substances such as tiny dust, and more particularly to an apparatus for inspecting foreign substances adhering to a substrate such as a photomask, or reticle for LS1.
2. Description of the Prior Art
In the process of manufacturing photomasks or wafers for LS1, foreign substances may adhere to a reticle or a mask and these foreign substances may cause a drawback of the manufactured mask or wafer. In the pattern printing apparatus of the type wherein the pattern on a reticle or the pattern on a mask is reduction-projected onto a mask or a wafer, a plurality of identical chips are formed on the surface of a mask or a wafer. Accordingly, this drawback appears as a drawback common to all of the chips on each mask or wafer and therefore must be strictly inspected in the manufacturing process. Thus, it would generally come to mind to inspect foreign substances with the eye, but this method would usually require much time for inspection and fatigue the operator, thus resulting in a reduced inspection rate.
In recent years, various apparatus for automatically detecting only foreign substances adhering to a mask or a reticle by applying a laser beam thereto have been devised. For example, a laser beam is applied perpendicularly to the mask or reticle and the light spot thereof is scanned two-dimensionally. At this time, the scattered light from the pattern edge on the mask or reticle (the edge of a light-intercepting layer such as chromium or the like) is strong in directivity and the scattered light from a foreign substance is created without directivity. There is known an apparatus for effecting photoelectric detection so as to discriminate between these scattered lights and inspecting, from the scanning position of the light spot, to what portions of the mask or reticle foreign substances adhere. With this apparatus, however, the whole surface of the mask or reticle is scanned by the light spot and therefore, in order to detect small foreign substances, it is necessary to reduce the diameter of the light spot and this has led to a problem that the inspection time is correspondingly lengthened.
Further, with this apparatus, it has been impossible to inspect the so-called adherence conditions of foreign substances, such as discriminating whether foreign substances adhere to the light-intercepting layer such as chromium or the like, whether foreign substances adhere to the glass surface (the light-transmitting portion), or whether foreign substances adhering to the light-transmitting portion lie on the laser light incidence side surface or the opposite surface of the object to be inspected.